1. Field of the Invention
The present invention relates to an electric power steering apparatus, and in particular to an electric power steering apparatus that detects a failure of a motor current detector.
2. Prior Art
An electric power steering apparatus, which energizes a steering apparatus of an automobile or a vehicle with torque of a motor, energizes a steering shaft or a track shaft with an assist force by transmitting a driving force of the motor using a transmission mechanism such as gears or a belt via reduction gears. Such a conventional electric power steering apparatus performs a feedback control of a motor current in order to generate assist torque (steering assist torque) accurately. The feedback control is a control for adjusting a motor application voltage such that a difference between a current control value and a motor current detection value is reduced. The motor application voltage is adjusted, in general, according to adjustment of a duty ratio Du of PWM (pulse width modulation) control.
Here, a general structure of an electric power steering apparatus will be explained with reference to FIG. 1. A shaft 102 of a steering wheel 101 is connected to a tie rod 106 of a steered wheel through reduction gears 103, universal joints 104a and 104b and a pinion rack mechanism 105. A torque sensor 110, which detects steering torque of the steering wheel 101, is provided on the shaft 102. A motor 108, which assists a steering force of the steering wheel 101, is connected to the shaft 102 via the reduction gears 103. Electric power is supplied to a control unit 130, which controls the power steering apparatus, from a battery 114 through an ignition key 111 and a power supply relay 113. The control unit 130 computes a current command value Iref of an assist command on the basis of a steering torque T detected by the torque sensor 110 and a vehicle speed V detected by a vehicle speed sensor 112. The control unit 130 controls an electric current supplied to the motor 108 on the basis of the computed current command value Iref.
The control unit 130 mainly includes a CPU (including a micro processor unit or a micro controller unit). A general function, which is executed according to a program in the CPU, is as shown in FIG. 2. For example, a current command value computing section 204 does not indicate a current command value computing section as independent hardware but indicates a current command value computing function that is executed in the CPU.
Functions and operations of the control unit 130 will be hereinafter explained. A torque command value Tref based on the torque detected by the torque sensor 110 and the vehicle speed V detected by the vehicle speed sensor 112 are inputted to the current command value computing section 204. The current command value Iref is outputted from the current command value computing section 204 and inputted to a subtracting section 206. On the other hand, a motor current Im detected by a current detector 205 is also feed-backed to the subtracting section 206. The subtracting section 206 calculates a deviation (Iref−Im). The deviation (Iref−Im) is inputted to a proportional-integral control section (a PI control section) 207. A duty ratio Du is outputted from the PI control section 207. The PWM control section 208 outputs a PWM signal to an inverter circuit 209 on the basis of the duty ratio Du. The inverter circuit 209 is subjected to PWM control on the basis of the PWM signal and supplies the motor current Im to the motor 108.
The above is an explanation of motor control for the electric power steering apparatus that is executed by the control unit 130. As described above, control for the electric power steering apparatus is performed on the basis of the detected motor current Im. Therefore, it is a necessary condition for the control of the electric power steering apparatus that a motor current can be detected correctly. Thus, when a current detector fails, it is necessary to detect the failure. When there is a separate current detector that should be compared with the current detector, it is possible to judge the failure by comparing an output of the current detector and an output of the separate current detector. However, when there is no current detector that should be compared with the current detector, the failure may be judged from outputs of sensors of other types.
For example, a Japanese Patent Application Laid-open No.2003-237609 A discloses a technique concerning miswiring abnormality detection in the case in which wiring to an assist motor is laid by mistake in a direction opposite to a normal direction. A detection principle for the miswiring abnormality detection is as described below. When wiring to the assist motor is laid by mistake, since a motor output becomes abnormal and a torque value to be detected oscillates. Thus, when a rate of change of the torque value increases to a predetermined value or more, it is judged that wiring of a current detector is laid by mistake.
The detection method in the document described above is a method for detecting miswiring. Permanent abnormality such as miswiring can be detected without a problem. However, in a case like failure of a current detector, an output of the current detector may show temporary abnormality rather than permanent abnormality. In such a case, there is a problem in that the abnormality cannot be detected with the detection method described above. In addition, failure is judged on condition that a rate of change of torque reaches a predetermined value or more. Thus, there is a problem in that it is extremely difficult to set a predetermined value for distinguishing oscillation in a torque value, which usually occurs, from oscillation due to abnormality of the current detector such that the predetermined value is not detected by mistake. In other words, misdetection is apt to occur.